In a known art, a die-cushion apparatus provided for a press has a structure which elastically supports a die-cushion pad disposed near a lower mold through a plurality of die-cushion pins and in which, when an upper mold lowering together with a slide abuts, through a work, against a blank holder to thereby apply a press load to the die-cushion pad, a pressure in a die-cushion cylinder supporting the die-cushion pad from the lower side thereof is discharged, and then the blank holder is also lowered together with the upper mold by an amount corresponding to a cushion stroke.
In the press die-cushion apparatus of the structure described above, a large colliding noise is generated when the upper mold lowering together with the slide abuts against the blank holder through the work.
For this reason, in a conventional technology, the die-cushion pad is preliminarily lowered at a speed lower than a lowering speed of the slide to thereby reduce the noise generated by the collision of the upper mold with the blank holder and hence to prevent the life times of the upper mold and the blank holder from being shortened.
In the conventional preliminarily accelerating device, the lowering speed is regulated by controlling a flow rate to a preliminarily accelerating hydraulic cylinder provided for the die-cushion by means of a servo valve.
Namely, a crank angle of the press is detected and when the crank angle reaches a preliminarily set angle, the servo valve is opened by a predetermined angle to thereby start preliminary acceleration, and when the crank angle reaches an angle at which the upper mold abuts against the blank holder, the servo valve is closed. Thus, the lowering speed control at the preliminary acceleration has been performed by an open-loop control mode.
However, in a case where the lowering speed control of the die-cushion is carried out by the above conventional open-loop control, large dispersion is caused in the lowering speed, and in the case of a large speed relative to the upper mold, such effect as the reduction of the colliding noise cannot be expected.
Further, also in a case where the die-cushion is lowered with a fast speed, not only a desired object cannot be achieved by the abutment of the upper mold against the blank holder after the preliminary acceleration, but also the normal press formation is not performed, resulting in a cause of production of defective. In addition, the setting of the angle at which the preliminary acceleration starts is to be decided through repeated trial formations, so that much time is required for the regulation of the lowering speed, thus being inconvenient.
Furthermore, in a conventional art, a die-cushion of a press utilized for drawing formation is composed of a hydraulic cylinder and a pneumatic cylinder in which pressurized oil is supplied by suction operation and the cushioning function is attained by a discharge pressure control of both the cylinders.
A die-cushion unit to be numerically controlled (NC) is connected to this hydraulic cylinder to thereby vary a cushioning capacity by controlling the servo valve for pressure discharge connected to a hydraulic cylinder in accordance with the crank angle of the press. A control unit e for controlling the servo valve has a structure, as shown in FIG. 6, for controlling a servo valve d by detecting a pressure of the hydraulic cylinder a by a pressure sensor b, comparing the detected actual pressure with a preliminarily set aimed value by a comparator c and outputting the thus obtained pressure deviation to the servo valve d by applying a constant gain to the pressure deviation.
However, in the press, the lowering speed of the press changes as shown in FIG. 8 in response to the crank angle, the lowering speed becomes zero at a lower dead point (crank angle of 180.degree.), and the slide speed changes in response to the operation speed of the press. Further, the characteristic feature of the servo valve d controlling the hydraulic cylinder a is non-linear, and accordingly, an object to be controlled by the control unit becomes nonlinear.
For the reason described above, in the conventional control unit, the actual pressure becomes dull near the lower dead point as shown by a curve B in FIG. 7 with respect to a pressure instructed value (aimed value) shown by a curve A in FIG. 7, and a pressure difference between the aimed value and the actual pressure value becomes large and the control performance is degraded, thus being inconvenience.